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Does the AIM really require anti-collision lights to be used during the day?

AIM 4-3-23. Use of Aircraft Lights
  a. Aircraft position lights are required to be lighted on aircraft operated on the surface and in flight from sunset to sunrise. In addition, aircraft equipped with an anti-collision light system are required to operate that light system during all types of operations (day and night).

Doesn't this imply that anti-collision lights are required to be on during the day? This is obviously not the practice?

Closely related

The AIM is not regulatory.  Anticollision lights are not position lights.

@acpilot the FAR is, though, and 91.209(b) requires them to be lit if you have them.

Yes, the far is.

Anti collision lights are beacons and strobes.  Normally, strobes are avoided when on the ground, except when taking the active.
Navigation or position lights are normally not run during the day, but on larger aircraft they are, and their intensity is greater.  The position lights on most planes are silvered lamps, and are quite pricey (~$40 US each when bought in quantity last I recall).
Generally, on light aircraft, best practice by day is to turn on the beacon prior to engine start, and leave it on until shutdown.  Nav lights are run during "dark" periods which includes adverse weather.  In bright sunlight, they offer little benefit, and operations I am familiar with don't run them.
On heavier aircraft, including biz jets, the standard practice changes, and the beacon comes on with nav lights at startup, and normally those lights run until shutdown.  When on a busy ramp at night, it is common practice to leave nav lights on to help bring attention to the wings and tail, even though the engines are off, and the aircraft may be stationary.
While you may not see it at an FBO (as their ramps are well lit, and they don't always have access to the aircraft interior), many freight ops and corporate flight departments will turn on the nav lights when tugging an aircraft on the ramp at night.
Many companies have lighting policies in their procedures which may go beyond this.
14 CFR 91.209 provides general guidance.

§ 91.209 Aircraft lights.
No person may:
(a) During the period from sunset to sunrise (or, in Alaska, during
  the period a prominent unlighted object cannot be seen from a distance
  of 3 statute miles or the sun is more than 6 degrees below the
  horizon) -
(1) Operate an aircraft unless it has lighted position lights;
(2) Park or move an aircraft in, or in dangerous proximity to, a night
  flight operations area of an airport unless the aircraft -
(i) Is clearly illuminated;
(ii) Has lighted position lights; or
(iii) is in an area that is marked by obstruction lights;
(3) Anchor an aircraft unless the aircraft -
(i) Has lighted anchor lights; or
(ii) Is in an area where anchor lights are not required on vessels; or
(b) Operate an aircraft that is equipped with an anticollision light
  system, unless it has lighted anticollision lights. However, the
  anticollision lights need not be lighted when the pilot-in-command
  determines that, because of operating conditions, it would be in the
  interest of safety to turn the lights off.

Anti-collision lights are required for day and night as specified in the AIM. There's an exception to turn them off if it interferes with the pilot's visibility (e.g. while in the clouds the strobes can be distracting).

This is obviously not the practice?

This is the practice.

Simplistic, but common practice...
Nav (aka 'Position') lights (low power red/green/white at the extremities of the aircraft... on when the aircraft is powered, to show it's liable to do something.
Anti-Collision (flashing/rotating red light, definitely on top of the aircraft, optionally also below the aircraft)... on when the engines are running, or just before. Warns people to stay away - ground personnel will not approach the aircraft while the red lights are flashing. (The rotating type are often switched off when flying in clouds to avoid visual disorientation.)
Strobe lights (high power, short burst, flashing white on the wingtips)... on in flight, and shortly before getting onto the runway. NOT used in general on the ground due to their high power (think pro-camera flash-gun, it's the same technology) possibly affecting other pilots' vision.
Taxi lights (low power "headlamps")... used when.... taxiing . (For some people, only when taxiing at night.)
Landing lights (high power "headlamps")... go on when "Cleared Takeoff" is received, or on entering the runway. Go off sometime after takeoff (gear up? 10,000ft? depending on operator). Are switched back on again (descending thru 10,000ft, gear down? cleared to land?) before landing.


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How can the CVR from a Cessna 560XL only contain old records and none from the fatal accident?
Is it possible that the CVR from a Cessna 560XL is just showing old records and nothing from the fatal accident when the plane crashed?
I´m talking about the airplane crash in Brazil, where a election candidate was killed. The investigation team said that the data in the CVR contained only old data and they could not pinpoint from when those recordings were.

Curious and Cris, I know having your answers (comments really) deleted might seem at first glance to be (another) example of elitest control over this site. It is not. Cris' answer was not a fully thought out and fleshed out answer to the question, which could stand alone by itself. It was a comment. If you will register yourself, and participate, you will soon enough gain the points to be able to comment successfully. Cris' comment should be made into a comment to ratchetfreak's answer. A question's author can always comment on his own question, so curious should have done so on his question

If the fuse was pulled between the previous flight and the crash then yes, the CVR can only record when it has power
Pulling the fuse or going behind the panel and cutting the wire will prevent it from recording.

@curious For one example case where the recorders only had old data, see here.

Recorders having old data can also happen if the erase head of a magnetic-tape recorder fails, resulting in an unusable mishmash of data from many different flights; for an example, see here.

The FDR and CVR are not magical, they can fail. So can the components of the aircraft that feed them data.
E.g. if there were something wrong with the microphones in the cockpit, the CVR would not record anything being said there. At most it'd record static.
Same if there were a fault in the data cable feeding the data to the recorder (I assume most of those circuits are at least dual redundant, but at some point it all comes together in a single plug).
If the CVR isn't properly connected to the aircraft, it won't record anything.
This can happen, and it doesn't have to be some malignant act. A simple error in a maintenance procedure can do it.
It wouldn't be the first time a flight recorder holds no useful data.
There have been other instances where the events that eventually caused the crash themselves caused the data feed to the recorders to be interrupted some time before the crash for example.

"...doesn't have to be some malignant act."  I think you meant "malevolent act", unless it was caused by a metastasising tumour.

Everyone
Fight Data recorder(FDR) and CVR(Cockpit Voice Recorder) are both present on the Cessna 560XL; The statement made by the authorities could:

Be true. But for that to have happened, the airplane would have to have both Circuit Breakers(CB) pulled out by someone. When pilots conduct their pre-flight checks lights would illuminate indicating the problem. Or.....
It is indeed part of a cover up. Several witnesses said they saw one engine on fire before the plane went down. Having an engine fire during a go-around maneuver inside clouds is something extremely difficult and the flying pilot could have lost situation awareness and became disoriented.

Hello Antonio, welcome to Aviation.SE. Please see our help center on how to write good answers. Can you possibly provide sources and citation for some of your statements, such as the CB being pulled and the eye witness accounts?

Having an engine fire "during a go-around maneuver inside clouds is something extremely difficult" ... that sentence seems to indicate you think an engine fire could be/was set on purpose, care to rephrase?

If you believe the eyewitnesses, every airplane that ever crashed had an engine on fire -- even gliders.

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What is this metal piece sticking up on the roof of a 182?
My CFI and I were stumped as to what the purpose of this metal piece is on the roof of a 182. I couldn't find anything from searching Cessna POHs/docs online... Any ideas?
Here is a close-up:

This appears to be an Outside Air Temperature probe.

Yep. Definitely not an antenna.

@JScarry - Do not put a Jack-in-the-box, styrofoam, antennae ball on it.

Correct, @DeanF., that goes on the larger antenna to the right in the 1st pic.

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What was the forensic process underlying the recent news regarding MH370 debris?
On or about July 30, 2015, the initial news could have been worded like: "debris appears to have come from a Boeing 777". As the story evolved, additional details emerged such as the wreckage being part of a 777 flaperon, before being decisively announced.
My question is: what was the forensic/investigative process which would have produced the initial speculative-sounding announcement associating the debris to a 777, before ultimately becoming the more definitive statement?
To put it another way: what steps had investigators taken up to the point of each announcement, and how was the debris recognized very early on as part of a 777 without actually being confirmed as such? Is it investigator experience? If initial identification was made by part numbers, why would there have been any doubt in the announcement?

A precise citation would be nice ("Medium X stated that Y on date D"), so that we can all reference the same exact wording.

How is this on-topic? I understand that it's about a part from a plane, and possibly about MH370, and even that it may be something akin to an NTSB investigation... but the investigative processes are not really aviation... just saying (please note that I did not down-vote or vote to close)

@CGCampbell Why would aviation accident investigation techniques not be on-topic? Perhaps some chat/meta discussion would be useful? Personally I would simplify the question to "how can accident investigators identify individual aircraft parts?" but that's just my take on it.

@CGCampbell I agree with pondlife whole heartedly, but this might be a good question for Meta Aviation Stack exchange, just to see how the whole community feels.

@Pondlife I think that the techniques are on topic, but media reporting of those techniques' results are not on topic. This question seems to be asking why the media has no clue what they're talking about when it comes to aviation, and the answer seems obvious.

@egid I agree with that, it's pointless to take media reports literally but I read the question more 'optimistically' as asking how do investigators arrive at a definitive answer? And as I mentioned, I think the question would benefit from being simplified to be more focused on the investigation rather than the reporting. So I think we probably agree on what is answerable here. I thought about editing it, but I hesitated because I might change the OP's intent too far.

My question wasn't really intended to focus on media reports, but on the information/statements issued/released by investigators, and the point at which hard facts end and inferences or speculation begins. I only mention media reports because they tend to be the most prominent conduit for this sort of information, accurate or not.

If you bring an NTSB investigator a piece of 2024 sheet aluminum with zinc chromate primer and a urethane topcoat all they can tell you is "It's 2024 sheet aluminum, with some paint on it." - BUT if you bring them a piece of 2024 aluminum riveted to a  structure that looks like this:

and tell them "I found it on an island in the Indian Ocean" they can infer  lot more.

In this particular case they've found some debris, and they know a few things:

The debris was found in an area where it's reasonable it could be part of MH370.
The location isn't conclusive in and of itself, but it's certainly a factor that strongly implies this could be part of the missing aircraft.
The part LOOKS like it came off an airplane.
It's aircraft aluminum, riveted to a skeleton that gives it an airfoil shape. That warrants further investigation by itself (even if we didn't lose an airplane in that area recently it's clear that some part fell off some airplane at some point in time, and investigators generally want to know which airplane had bits falling off).

That leaves us with the question Could this part have come from the airplane we lost? which is the stage we're at now, and there are many ways to make this determination.

You might get lucky and recover the aircraft ID plate, or a chunk of the tail with the N number.
There's confirmatory testing to be sure it's not a well-perpetrated hoax, but these are pretty definitive identification elements. Unfortunately that's not the case here, so we move on.
If you recover what appears to be a control surface you'll usually find a manufacturer stamp somewhere on the part, or a serial number plate. They'll certainly be inspecting this debris to look for that kind of data tag. (The data tag below is from a Piper control surface but Boeing probably uses similar plates).

If you don't find a data tag maybe there's airline livery on the debris.
This at least lets you know whose plane you've found, but that's not applicable here as the debris in question appears to be painted basic white.
Failing all of that you compare what you've found to manufacturer's drawings of the aircraft you think it came from and see if the dimensions match any parts or sections of that aircraft.

There are other possible steps and confirmatory testing involved (metallurgy, mass spectrometry on the paint, etc.) to determine if it's likely this part came from the aircraft you think it did, but that's getting into the territory where you'd be better off signing up for training courses in accident investigation.

Of course, in this particular case, if they can confirm that it is indeed a 777 flaperon, then they've also effectively confirmed that it was from MH370, since only 3 other 777s have ever crashed and none of the others were anywhere near the Indian Ocean (and all 3 were on land, meaning that their flaperons were recovered immediately... in two of those cases because they were still attached to the almost completely in-tact wings.)

@reirab Yup, the 777's excellent safety record makes it much easier to figure out which crash any given piece of debris came from :)

@voretaq7 I'm interested to know how, in this case, a piece of debris not confirmed to be from a 777 can be declared as "yup, looks like it came from one of 'em", without it being pinned down as "part x from aircraft model y". Yes, the piece in question looks to me quite obviously like an aircraft part, but damned if I could associate it with a specific aircraft type. Without solid confirmation, what allows an investigator to so quickly rule out other aircraft types? Why are they able to rule out older incidents involving other aircraft?

@AnthonyX the exact shape and size, that you cannot infer from a photo, but that investigator can measure and compare to the original designs.

This from a CNN article on the debris:   "The source said Boeing investigators are basing their view on photos that have been analyzed and a stenciled number that corresponds to a 777 component."   They didn't indicate who those "sources" were.
Another source told CNN that Boeing engineers have seen a part number -- 10-60754-1133 -- in photos of the component. A Boeing parts supplier confirmed the number was on a seal associated with the Boeing 777, the source said.
Images of the debris appear to match schematic drawings for the right-wing flaperon from a 777.

@AnthonyX Also, only a few aircraft types even have flaperons at all and this one is the same dimensions as the one on the 777. That narrows it down a lot, even before confirming with part numbers and such.

The Aviation Herald now quotes Malaysia's Transport Ministry statement (on Aug 2nd 2015) that the part has been confirmed to be a flaperon from a B777 and that they are now trying to verify whether it was indeed from 9M-MRO.

AVHerald now also includes a pick of a serial number.

They know the debris is from a 777 because the parts have part numbers on them which are unique to that model.
Since all other 777s are accounted for and no other 777s have crashed in the Indian Ocean, the flaperon ccould only be from MH370.

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What is the maximum altitude a skydiving plane can fly?
Is there a standard maximum altitude which skydiving planes can fly?
I am mainly asking about the sports activity, not military parachuting.

The maximum altitude the plane can fly at, or the maximum altitude at which people can jump out?

135,889 ft. source.

@RaajTram its not by plane but by a balloon....

Qualifier: I am a master parachute rigger and taught sport skydiving for 10 years.
Standard jump altitude is 12,000 feet AGL - this is one turn of the standard parachutist's altimeter. This height provides 1 minute of freefall, is within reach of a Cessna with a decent engine and does not require supplementary oxygen. For most places, this means an operating altitude between 12,000 and 15,000 feet MSL.
Dropzones in high areas like Denver will reduce their jump altitude to 9,500 feet AGL mainly because the piston-powered planes take too long to climb higher and/or the upper part of the climb runs into the oxygen zone.

ah so ain't 9,500 feet is too low and you wont get even 30 sec of free fall?

@NitinG - if you get 60 seconds of freefall at 12,000 ft, wouldn't you get around 45 seconds of freefall at 9500 ft?

45 seconds from 9,500' is exactly what we write down. Nobody carries a stopwatch, it's just standard times from standard heights.

I'd rather ride a C-135 to 45,000 feet and get a minute and a half of free fall - without a parachute.

As paul mentioned, "standard" altitudes for sport skydiving facilities near sea level are in the 10,500-13,500 AGL range.  Dropzones significantly above sea level typically either reduce their jump height to stay under 15,000 MSL or provide supplemental oxygen in the plane and go up to 18,000 MSL.
Many dropzones will offer jumps from 18,000 MSL for an additional fee over the regular jump.  Supplemental oxygen is provided.  Jumps in the 18,000-23,000 MSL range are also possible but rarer because of the increased needed for coordination with ATC.  Above 18,000, the risk of hypoxia increases rapidly.  This video shows jumpers suffering from severe hypoxia at 21,000'.
There is at least one facility in the United States that takes recreational skydivers up to 30,000 MSL.  It requires additional training and wearing oxygen in freefall.  This facility uses a King Air, and I know it has also been done from a PAC750.

You can skydive over the Frans Josef glacier in New Zealand from 19000 feet. And similar other high altitude jumps around AU/NZ. I wouldn't call it rare (maybe in the US?)

If there's such a thing as a standard maximum altitude for sport parachuting, the best answer would be 15,000' MSL, as above this CFR 91.211 requires supplemental oxygen for all passengers - and providing this to sport jumpers would be an expensive nuisance.
Note that this does not vary with ground elevation - so indeed jumpers in the Denver area will not be able to get as high above the drop zone (and thus as much freefall time) as those starting from a lower altitude.
Note also that this is a limitation on people - there is no standard maximum for jump aircraft  Some aircraft types (e.g. Cessna 182) would struggle to get anywhere near 15,000' MSL in a reasonable time. Others (e.g. Cessna Caravan) can haul a full load of jumpers to 15,000' MSL rapidly and without breaking a sweat.  The Caravan could carry jumpers to its service ceiling of 25,000' provided they had oxygen and the pilot obtained ATC clearance (required above FL 180 = 18,000').

Looking at CFR 91.211, I don't think it's the same rules elsewhere in the world. We jumped from 14000 ft in Australia and 19000 ft in New Zealand without oxygen, and without a pressurised cabin (for the passengers, no idea about the pilots)

Paul said "Standard jump altitude is 12,000 feet AGL -This height provides 1 minute of freefall, "
that is incorrect actually...
Basic skydive has an exit altitude of 10,000 feet. This is the average altitude for skydiving here in the US and common to most Cessna aircraft dropzones.
This means you will skydive from 10,000 feet above ground level.
At this height, your skydive will last 30 seconds - that's how long you'll be in freefall. You'll then have a few minutes under parachute as you make your way to the ground.
EXTREME SKYDIVING ALTITUDE: 14,000 FEET
When you jump from 14,000 above ground level, you'll be in freefall for around 60 seconds.
THE HIGHEST DAILY JUMP is: 18,000 FEET
At a skydiving altitude of 18,000 feet, skydiving lasts for 2 minutes
Any way the correct answer is
basic skydive from 10,000 feet above ground level.
At this height, your skydive will last 30 seconds

The real answer may be what configuration one is in during their dive, which would determine their terminal velocity. At 120 mph, you drop a mile every 30 seconds.  I would defer to experts the safe minimum altitude to open the chute.  But falling arms out belly down would be a bit longer than diving head down!


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Do nacelles around aircraft engines help in reducing the engine sound?
I know that the nacelles around the aircraft engines are actually a housing and are generally lightweight and have may components like inlet cowl, fan cowl, etc., but do they help in reducing the engine sound as well or are they just for housing?
Even if they help in reducing the sound, then how much % of sound has been reduced by them?
Is there any quantitative analysis available for that which can show that without housing engines were creating sound at X dB, and after nacelle and proper housing of all other components, the sound has been decreased to Y dB?

How much is what? Can you please clarify?

edited with more clarification

Take a modern turbofan: The tips of the fan run at approximately Mach 1.5. We know from unducted props that supersonic tips make them very noisy. The same turbofan without the shroud would be extremely noisy, too. Acoustic liners on the inside of the fan shroud have major contribution in suppressing this noise source.
But there is more: The nacelle also makes it possible to put an intake in front of the fan which helps to equalize flow conditions over the cross section of the intake and to accelerate or slow the flow of air to just the right amount for the engine. Without the intake, the engine would suffer from frequent compressor stalls and surges, so the nacelle in effect widens the operating conditions and avoids those noisy incidents, too.
Another important source of noise is the nozzle: Here the hot, fast gas from the engine is ejected into the surrounding flow, and the mixing of both creates most of the engine noise. Note that noise increases with the sixth to eighth power of jet velocity, so the most dramatic noise reduction is possible with higher bypass ratio engines.

Diagram from the Stanford web page linked above. The jet velocity is in ft/s.
By serrating the trailing edge of the nozzle, this mixing process can be slowed down somewhat and the noise level can be reduced.
Without the nacelle the engine would create much less thrust and more drag, and bigger, more powerful engines would be needed. Bigger engines would create more noise, too, so this is also an indirect way how nacelles help to keep noise down.
A last noise reduction contribution to mention here is the capacity of the nacelle to contain the shrapnel from a failed fan blade. Fan blade failure is all but silent, but if the blade fragments would escape at high speed, the noise of what follows next would also be considerable.

Regarding your last paragraph, this one was a turbine disk rather than a fan blade, but it did generate quite a lot of noise in the form of discussion in the cockpit, radio chatter, media reports, and a few years of subsequent discussions about why it happened.

As you have correctly mentioned, the term Nacelle generally refers to a number of sections around an aircraft engine (In the context of turbofan aircraft engines). This generally the structures formed by the Inlet, Fan Cowl, Thrust Reverser and the Exhaust.
The nacelle does perform the basic function of a housing for all these subassemblies which means it contributes to protection of the engine from the elements and the environment from the engine (Say a blade failure). But its major function is to provide an aerodynamic body which can house the mechanicals of the engine, the pneumatic ducts, fuel lines and electrical wiring.
The nacelle can therefore provide noise reduction by reducing the drag-noise which would be otherwise generated. This is through the aerodynamic design and the various features built into the subassemblies such as Acoustic Liners used extensively througout the nacelle which convert acoustic energy into small amounts of heat. This is a major source of noise-reduction in modern aircraft engines.
Exhausts are the biggest culprits when it comes to aircraft engine noise and as such, the exhausts are designed to create a 'mixing' area where hot air from the exhaust is mixed with ambient air to reduce the exhaust noise.
I guess I'm just trying to say that the nacelle is formed of these various subassemblies and therefore their individual contribution towards noise reduction is very important. As far as a quantitative study on the reduction of noise, I could not locate any as therefore can only manage to answer the first part of your question.
Sidenote: Modern Inlets also use Laminar-flow design (777X for example) which contribute to immense fuel savings and very minor drag-noise reduction (Probably negligible).

The nacelles themselves reduce sound just by being another barrier before the sound gets to you for the most part. It's there to provide airflow around engine components, better aerodynamics, protection from elements, and depending on what kind of engine it is, it "may" help with noise reduction some (Turbofan).
On turbine engines, there are exhaust airflow options that do suppress the noise. This is not a nacelle, it's a noise suppression nozzle.

You are mostly right, but there are multiple design features in a modern nacelle that play a large role in noise reduction.


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Does weight and balance of an airplane matter more on takeoff?
On a few international flights with non-full cabin, the crew has asked no one to move to open seats until the plane has finished take-off. I assumed this was just to force people to sit down during take-off and not cause problems, but when I asked I was told it was because of weight. The claim is, the airplane is balanced by where people are sitting. The next claim is by the end of the flight, there is less fuel, so if people have switched seats there is less of a problem. This weight balance apparently uses all information, people, baggage, and fuel.
I believed this, until recently a plane had lost at least 30 peoples baggage, some people had more than 1 bag. The bags were left at the previous airport, and on arrival no one knew that the bags were not on the plane.
If the lack of knowledge for bags not being on the plane (which seems logical, why would they depart without the bags), how did this change in weight not show in the take-off, or was this a dangerous occurrence?

This sounds like a specific question about a specific situation.  What kind of airplane was it?

By regulation, a "weight & balance" must be computed before departure to ensure that the weight is within the weight limits for a number of different conditions and that the weight is distributed such that the c.g. (center of gravity) of the airplane is also within the longitudinal c.g. limits for those conditions. That c.g. is typically computed assuming the the passengers are in the seats they have been assigned. If the passengers are seated elsewhere, the computed c.g. is not accurate. In other words, the pilots wouldn't know where the c.g. is or whether it is even within limits.
The takeoff trim setting is affected by the c.g. If the pilots set the trim according to the computed value, and the c.g. is considerably aft of that, on liftoff the nose is going to want to pitch up more than the pilots expect. If the c.g. is so far aft that it is well beyond the aft limits, serious control problems can occur. If the c.g. is far forward of what the pilots are expecting, they're going to have to pull harder on the elevator control than expected to get the aircraft to rotate, and that is going to extend the takeoff roll.
Different makes of aircraft vary widely in c.g. limitations and how tolerant they are limit wise of passenger placement in aircraft that are not full. For example, when I was flying Metroliners, if you had few passengers, you had to ask them to sit toward the rear. I once got a strange look from Rosey Grier (Google him if you don't know who he is) when I had to ask him to sit in the back of the aircraft.
Once you're in the air, the movement of passengers is of lesser concern balance wise. You're at speed, the elevator is fully effective, and the pilots or the autopilot keep the airplane in trim. In other words, the trim is what it is rather than a computed value that might not reflect reality.
Also, on large aircraft, once you're in the air in level flight and if you're so inclined, you can work out your weight by using your fuel flow, pressure altitude, and airspeed. Then with the weight and your trim setting you can come up with your c.g. I would guess that most of the glass cockpit aircraft probably do that for you automatically if you request it. My experience stopped with the 747-200, which is now a near relic.
The c.g. limits are defined conservatively, and usually in terms of the percent of the mean aerodynamic chord. It's not a case of that if the aft c.g. limit is 33% mac (a common value for 747s) and you're at 33.1% you're going to crash. It's just a case that you're beginning to move into an area that gives you a lesser degree of controllability than is deemed necessary for some situations. Continued movement to an extreme value would eventually make the aircraft uncontrollable.
Insofar as the bags-left-behind part of the question, I don't think their absence would cause a problem. If the computed c.g. including the bags was right right up against the aft c.g. limit, their absence could conceivably put the c.g. beyond the aft limit. The pilots might notice it, but handling it would not be difficult. However, aircraft are typically not loaded such that their c.g. is close to a limit. A preferred aiming point for 747-100s, -200s, and -400s is 26.6% mac.
Just for the heck of it, I did a weight & balance for a 747-400, loading it so that the takeoff c.g. was 26.6% mac. I then took 5000 lbs (100 bags x 50 lbs each) out of the most forward baggage area. This changed the the c.g. to 28.0% mac, still within limits. On the 747, a given weight of baggage misplacement has less potential to affect the c.g. than the same weight of passenger misplacement because the passenger seating extends over 20 feet forward of the most forward baggage compartment and over 17 feet aft of the most aft baggage compartment.

Excellent answer, always love to read your stuff Terry, appreciate it!

If the absence of 100 bags doesnt matter, why does passenger seating?

100 bags at less than 50 lbs each matters as much as 34 passengers at 150 lbs each.

It's winter time in the Northern Hemisphere. So passengers are considered to weigh more. The current average used is almost 200 lbs. So you're really down to about only 25 passengers == 100 bags. :-)

And passenger seating matters because a couple of hundred lbs of passengers sitting in the far back row could make a pretty big difference in the CG of the airplane, even in a larger airplane.

What @Shawn says seems very relevant to me. The passenger cargo hold is just a small section of the planes cargo deck, and usually is quite close to the cg, whereas the passengers are spread all along the plane.

@user1938107 Another senior moment on my part in that I really didn't word the last part of my answer as well as I should have. I've expanded that last part to hopefully explain things better.

In the movie Speed it was a bus, but remember all of the passengers had to shift to one side so they would make the turn?  Same basic idea...that is if you were having trouble visualizing anything that's already been said.

@BaileyS I'm not familiar with Southwest or the 737 so can't address those, but I'm aware of 3 ways to handle this. There are others. Remember, the question to be answered is whether the CG is within limits, not so much that you know its exact position. Therefore computerized systems can work two solutions, one with everybody forward, one with everybody aft. If neither is out of limits, you're safe. Another way is estimating the number of pax in "zones." Another, mainly for manual systems, is "curtailment" of the limits. I suggest asking this as a regular question to allow room for answering.

Regarding the core of your first question, takeoff is a sensitive "critical phase of flight" where center of gravity can affect the aircraft's ability to rotate. Generally takeoff has the tightest margin of safety, as you are at your heaviest. This affects your ability to rotate, as well as your ability to stop safely should there be a malfunction on takeoff.  The movement of people will affect the aircraft's ability to climb out safely.
If you want an extreme example of how center of gravity is important, watch this video:  - the cargo broke loose and fell aft on takeoff causing the aircraft to pitch up out of control.
For landing, it's generally just important that your center of gravity be within a certain window, which is larger after you've spent all of the fuel over the course of the flight.
Regarding the bags, that could have happened for several different reasons. Generally speaking though, the lack of bags would probably move the CG forward, which will probably make the flight safer (again, depending on the circumstances).

Controlling where people sit is about balance of the airplane. Moving weight changes the dynamics of the aircraft around the center of gravity.
One way to look at that is by thinking of the scale at a doctor's office. The nurse moves the small weight left or right until the scale is balanced. Where the nurse moves it teeters the balance. Too far in either direction and you could hop up and down on the scale and not make it move.
Leaving baggage is an overall weight thing. If the airplane is overweight, they have to leave something behind. And they'll boot bags before they boot passengers.

The airplane must have enough power and "lift" so that it can clear any obstacles at the end of the runway.   The more weight at takeoff, the more lift is required.   The "lift" of an aircraft is dependent on temperature, barometric pressure, and humidity.  These things affect the density of the air.  And, the lower air density produces less lift, so the total weight of the aircraft must be reduced to insure a safe takeoff.

While this is correct, this is not about the topic asked in the question (passengers asked to use their assigned seats to maintain weight centering, but luggage not aboard seem to be less important)


text:
How to calculate the force exerted by pressure on hot air balloon envelope fabric?
How to calculate the force of hot air balloon envelope fabric?
and how to select a fabric which can bear this force?
For example, a spherality balloon:

$r=10~m$
$T_1=100~°C$
$ρ_1=0.95~kg/m^3$
$T_2=20~°C$
$ρ_2=1.25~kg/m^3$
$m=1000~kg$

Where:

$r$ is the  radii of the balloon, in m
$T_1$ is the temperature of balloon inside atmosphere, in Celsius
$ρ_1$ is the density of the balloon inside atmosphere, in  kg/m3
$T_2$ is the temperature of balloon outside atmosphere, in Celsius
$ρ_2$ is the density of the balloon outside atmosphere, in  kg/m3
$m$ is the total weight of the balloon(including balloon all equipment and persons)

Hi! As this stands it sounds like you're trying to get us to do your homework. Could you perhaps elaborate what you have tried to do to solve the question on your own? You're also best of splitting this question (forces/materials) into two separate ones.

This looks more like a physics homework than anything else.

related: http://physics.stackexchange.com/questions/142697/the-physics-behind-balloons-lifting-objects/142698#142698


text:
Do fighter jets have forward-looking radar warnings for missiles, and when was this first implemented?
Let's imagine a swarm of fighter jets intercepting another swarm. Both groups are moving head-on towards each other. Once they get in range, they will probably lock on and fire missiles.
Do fighter jets have radar that will show and warn of a missile coming head on? Or do they only scan their tails?
I'm interested in when, if ever, such capability was implemented. To me it seems like the 50's and 60's and maybe even the 70's, they would only worry about scanning their tails. It would also be good to document this for both NATO and Warsaw Pact jets.

Radar Warning Receiver systems (RWRs) used by military aircraft are omnidirectional. However, they do not warn of incoming missiles (unless that missile is at least semi-actively radar-guided, like an AMRAAM). What they warn of are various patterns of radar activity, including a radar lock (which is a constant, high-intensity scanning of the aircraft's position, more than just a "blip" as the antenna passes by on a wide-angle scan pattern).
If another aircraft has a radar lock on you, you should be expecting an incoming missile of some kind. However, IR missiles (AIM-9 Sidewinder, AA-11 Archer, MICA IR, ASRAAM) do not emit any EMR that indicates they're incoming; they use a passive FLIR sensor to identify and track the heat source they were told to kill (they don't even require a radar lock; the seeker can be cued to a pilot's helmet, or it can be "uncaged" and will lock on to the most significant heat source in front of it). Passive radar-guided missiles like the old AIM-7 Sparrow also do not emit their own radar; they track the radar returns from the firing aircraft. Only semi-active and active radar-homing missiles emit their own radar noise that can be used by the RWR to know one is on the way. A few older passive missile systems used radio control to guide the missile to the target, and RWRs can also detect these "sideband" radio signals as a clue a missile has been launched.
More modern Missile Approach Warning Systems (MAWS) have additional features that can help detect and track incoming missiles of multiple types. The biggest clue of a missile launch is a new heat signature, especially in the direction of an incoming radar lock; a suite of IR sensors can be used by MAWS to put 2 and 2 together and warn you of the missile launch.

Can you tell me if RWRs, as you defined them, have been common to NATO and WP fighter jets ever since WWII? Also, can an IR missile be fired head-on? I figured they need to chase the heat exhaust (tail) of a jet.

The first U.S. aircraft to be equipped with RWRs were Vietnam-era fighters as a countermeasure against Soviet-built NVA SA-2 sites. The first ones in theater were F-100F Super Sabres as a trial for a new "Wild Weasel" strategy, whose success led to follow-up Wild Weasel variants of the F-105 and F-4. B-52s got a similar system around the same time. RWRs were incorporated defensively into other Phantom and Thud variants by the end of the war, and became standard issue on the F-5 and all subsequent aircraft.

@DrZ214 While the engines obviously give off the hottest heat signature, the rest of the airplane is warm relative to the cool air around it and head on IR missile shots are a possibility.

@DrZ214 - Rhino Driver is correct about IR missiles. The technology is called "all-aspect lock-on" and it became available to U.S. pilots as of the "L" variant of the AIM-9 Sidewinder beginning around 1977. Practically all IR missiles now in active service are all-aspect.

@RhinoDriver Thanks for the radar info. I opened a related question about the IR stuff: http://aviation.stackexchange.com/questions/18775/how-do-combat-jets-know-when-to-launch-flares-to-counter-ir-missiles

"IR missiles (AIM-9 Sidewinder, AA-11 Archer, MICA IR, ASRAAM) do not emit any EMR that indicates they're incoming" - Strictly speaking the thermal energy output by the engine is EMR.  Theoretically at least if you could detect that there's something unusually hot moving unusually fast towards your position then you'd have an indication of the incoming projectile.


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What is the response time of Pulse Doppler Radar?
We have the Airborne Pulse Doppler radar for weather predictions which are I think same as in the A350s and also in fighter jets so, what is the response time of these radars?
For example, if a fighter jet is travelling greater than Mach 1, how much will be the response time of the weather radar and how the response time will affect w.r.t to the speed of the plane?
Edit: Response time means how frequently the data coming from the Radar is getting updated? Like in case of Flight Radars etc, the normal response time or update time for the data varies from 40ms to 100ms.

Could you please clarify what you mean by response time?

@DeerHunter question updated

Isn't it usually called scan rate?

Depends on the radar and the mode it's in.

If you are thinking about how long it takes for the pulse to hit the target, be reflected and received back at the radar, the speed of any aircraft is so tiny compared to the speed of the pulse, that the error introduced would not even be measurable on the resolution of the display.  You are talking about an error of about 1.4e-6.

We have: what does the manual say?

As Simon alluded, the aircraft's speed (example given Mach 1, approx. 340 m/s) is basically one-millionth the speed of the radar energy (traveling at near-light speed, or 300,000,000 m/s).  It's a non-factor.
Ideally, if you want to radar detect something (water droplets, for instance), the detected object needs to be bigger than one quarter-wavelength of the signal you are bouncing against it.  This alone dictates that the weather radar should operate in the microwave range, typically 8-12 GHz (quarter-wavelength approx. 6-10mm) for most systems.
Radar signals are sent out in pulses a few μs wide.  The pulse frequency is determined by how far out you are scanning.  More distant weather requires a longer "wait" time before a second pulse is sent out.  For instance, a pulse detecting weather 5km out will return about 35μs later (10000m round trip divided by the speed of light), but typically systems scan much further.  100Hz (pulses per second) pulse repetition frequency is not uncommon (i.e. every 10ms).  This is good enough to provide location only.  Pulse-Doppler radars provide an understanding of object movement (i.e. wind speed and direction) as well as location.
We also need to take into account the antenna scan time.  Because the radar signal only returns objects directly in front of the where the antenna is pointing, the antenna needs to be moved around so as to pick up a greater area, and so the scan time is largely affected by the scan pattern.  (Aircraft pitch/roll is also accounted for by most weather radar systems, though depending on the gyros, it is not uncommon for the radar to paint the ground momentarily during a maneuver.)  The Honeywell RDR 2100 takes 3-4 whole seconds to perform a full left-to-right scan (typically 90 degrees or more).
Scan time is also affected by the beam width.  Smaller antennas cannot focus as well as larger antennas, and so will produce a larger beam width, requiring fewer scans to produce a complete picture, at the expense of resolution.
Transferring all this weather data around takes additional time.  Commercial aircraft systems do this over an ARINC 708 data bus at 1mbit/s.  Range data, intensity, antenna azimuth/elevation, transmitter gain, information is all sent to be processed and displayed.  Higher resolutions will take longer to "paint" the screen than lower resolutions.  The actual data transfer time and display processing time is negligible, because your cockpit indicator might only have a refresh rate of around 60Hz, or a full screen update every 17ms, with a pixel response time less than 5ms.  For reference, this is the same refresh rate as a typical flat screen TV.
All told, from the time a radar pulse is sent out, detects weather, returns, is processed, and displayed on the screen, the time elapsed is typically no better than a half second, and typically longer.


text:
How much power is needed by aircraft IFE systems during flight?
How much power is needed by aircraft In Flight Entertainment systems during flight if they operate all the devices connected in the cabin?

Could we perhaps make the cabin lightening a new separate question? Makes it less broad since it shares little in common with IFE.

Done with the changes

thanks for doing that, it helps keep information and content a bit orderly when looking for stuff later :)

I don't have specifics but lets extrapolate for a minute (using the Boeing 777-300 for size filled with lets say 500 seats). Assuming you are talking about IFE systems where there is a screen in every seat then you will most likely have screens similar to this. While not exactly what the airplane uses I would think they are similar. The specs for that screen claim it takes <= 8W. Lets round it out and say it takes 8w. Looking at our 500 seats thats 4000W of power. I can only assume there is a computer or two of sorts running those displays, lets say they have 500W (typical desktop supply on the more powerful side) and there are 2 of them. Thats another 1000W so you are at 5000W there. Assuming you have 120V power P=IV thats about 41 Amps of current you will draw from the system. I am running this assumption that all the screens are on at all times as I assume the system is built for that.
Cabin lighting is tough as I have no idea how many bulbs they have in there. I know that each seat has its own little lamp so you are looking at 500 little lamps running in the 1W-10W range. So maybe another 2000W for the individual lights. Overhead is tough since you cant really see the individual bulbs. It should also be noted that LED's are becoming more wide spread and draw way less power than traditional incandescent bulbs which will greatly reduce the amount of power drawn by the interior lights.

It depends on how many lights are on, and how much the in-flight entertainment system is in use.
Cabin Lighting
New aircraft exclusively use LED lights1 for cabin lights. Compared to incandescent or fluorescent bulbs, LED bulbs much less power. If I'm not mistaken, LED bulbs are being used on aircraft lighting (at different levels) for 20 years or so.
In-flight Entertainment
Power consumption of IFE systems is mentioned here:

Most IFE system PSUs fall into the power range of less than 100W,
  although some newer IFE products require higher power solutions of 300
  watts, or more.

Please note that an IFE system is not just the monitor a passenger sees, but it has other hidden components too, which require power as well.

1 Biggest example is A380:

The A380’s interior illumination system uses bulbless LEDs in the
  cabin, cockpit, and cargo decks. The LEDs in the cabin can be altered
  to create an ambience simulating daylight, night, or intermediate
  levels.

I had a look around but couldn't find too much. This is a competitive industry so they would be inclined not share too much information (a bit like fuel and jet aircraft). The power consumption is probably the top of the iceberg of important things to consider and I imagine manufactures don't want to make their products look unattractive without showing the customer the full scope of what they are buying, such as software capabilities.
After much searching, Rockwell Collins were kind enough to publish (dated 2013) some information for overhead monitors systems. The media server uses about 80 watts (so a bit more than an average laptop) and the interactive screen to control the content about 20 watts. There is no figures on how much each actual screen uses. Be aware this is for a system that shows the same content on all screens.
I found to this document that suggests that the power supply draws around 100 watts (that would fit with the suggestion above). Be aware however that it was written in 2008, the year after the first iPad was released and tablet computers (and corresponding expectations) took hold in the public. A lot has happened since then and as stated in the paper newer systems can draw a lot more. Furthermore, power supplies can be shared among multiple seats (the one shown in the paper has "multiple outputs"). This quite extensive paper also discusses IFE design.
The competition also means that they would like to make the devices as light and efficient as possible. Looking at the X-Series from Panasonic ("The world's most popular in-flight entertainment system" quoting themselves) the screen does video decoding and also includes Android compatibility. Aside from the substantially higher price tag, I think it would be a fair estimate to compare the power in the high range for a tablet computer.
It's always good to contrast these figures: If you're going to go providing AC and USB plugs, laptop chargers use in the range of 80 watts and half the passengers on a A320 use a USB plug for their charging their empty gadgets, you're looking at 350 watts.


text:
How much humidity is being maintained in cabin and cockpit of B787?
I think in general there is less than 10% humidity is being maintained in most of the commercial aircraft but I think they have improved that in the 787. I am not sure how much that is and if the cabin and cockpit have same amount of that?

These sources say that most airliners maintain a humidity of 4-7%, while the 787 maintains 10-15%. Of course these values can vary with conditions, and other sources put them lower or higher.
The cabin and cockpit will not have very different air qualities. While some cockpits may humidify the cockpit air slightly, the cockpit tends to have a greater amount of circulation, leading to drier air than the rest of the cabin.

You beat me to WSJ!!!


text:
what are the ways to avoid skidding off the runway apart from thrust reversers?
what are the different ways to avoid skidding off the runway apart from thrust reversers?
Do crew have any option if thrust reversers are not available and the ground is too slippery which is making the vessel skids off the runways sideways?

Why do you assume thrust reversers aid directional control?

Thrust reversers are often the reason why an aircraft went off the runway.

Your question appears to assume that thrust reversers are an acceptable way of keeping an aircraft from sliding off a slippery runway. While this may be true in rare cases, generally speaking you would NOT want to use reverse thrust to avoid skidding off a runway as that typically would exacerbate the problem.
Consider the following scenario: you have an icy runway and a strong crosswind from the right. As you start the takeoff roll, the airplane begins drifting left. You apply right rudder to correct, and the aircraft heading moves to the right of the runway direction. If you resolve the engines thrust vector into it's components relative to the runway centerline, you have most of the thrust propelling you down the runway, a much lesser amount propelling you toward the right side of the runway.
Lets say the left drift continues in spite of the engine thrust vector driving the aircraft to the right side of the runway, and you decide to abort the takeoff using reverse thrust. Thus, the thrust vector relative to the runway centerline will slow the aircraft. However the thrust vector 90° to the runway centerline will be toward the left side of the runway, which when added to your leftward drift will likely put you off the left side of the runway.
If you Google "tower air 747 crash" you'll see a number of entries concerning just such a scenario and the result. I found http://flightsafety.org/ap/ap_mar97.pdf to be the most instructive, although I do not wholly agree with the conclusions or with the narrative of what happened (a friend of mine was in the jumpseat right behind the captain, and I flew as a f.o. with that captain).
An obvious answer to your question is to never get yourself into such a serious situation. Can you wait for better conditions? Is another runway available? But if you MUST use a slippery runway for either takeoff or landing (or have already foolishly committed yourself), remember that if a runway is that slippery, it's feasible to continue down the runway in a manner that your course (the runway direction) doesn't match your heading. If the captain of the Tower Air flight had elected to do that, he might not have had the accident he did, or he might have had a much more serious accident. Tough call!

Thrust reversers are not used to steer an aircraft.
On the ground the rudder is used to steer during landing and takeoff. The main wheel brakes can be used to make tight turns when taxing.

The aerodynamic controls (ailerons, elevators and mainly the rudder for this question) still have an effect when the aircraft is on the ground but is still at speed. So even if the plane slides or aquaplanes a little bit at high speed it is generally controllable.
The problem is at lower speeds when these controls don't work as well / at all. You could be in a situation where the aerodynamic controls don't work and you are going to fast for the wheels to grip, or even you are stopped and a crosswind blows you sideways off the runway.
If the wind is down the runway hopefully you can get down, but taxying might be another matter. Possibly there are reduced cross wind limits on icy runways... I don't know I only fly light aircraft and generally not in those conditions.


text:
Why aren't there more helicopter aerobatics at airshows?
I enjoy watching RC helicopter stunts like these.  Many RC pilots are very talented, executing barrel rolls and other inversions.
How come we don't see these kind of displays at airshows?

After watching the video from ratchet freak's answer, I (personally) think that airplanes doing aerobatics are way more "majestic looking" than a helo.  But it is interesting that there aren't more aerobatic helo displays.

You may want to take a look at the Royal Navy's Black Cats.

First off: I'm not a pilot.
I believe there are two additional issues here:

There is no pilot in the RC helicopter. The forces exerted in some of those maneuvers look pretty extreme, I highly doubt even the most well-trained aerobatics pilot/astronaut/fighter jock would be conscious longer than about 2 minutes into the video you posted. I'm not even sure if they would be able to survive that.
Scale. All sorts of weird things happen when you scale complex systems up or down. (A famous example is the square-cube law: when you scale a structure up linearly, it's area scales as a square, and its volume (and thus weight) scales as a cube. So, an ant that is 100 times bigger will have feet that are 10000 times bigger but will have to support a weight that is 1000000 times bigger.) For example, the airframe of the RC helicopter is much stronger in relation to its size than it is on a real helicopter. Also, RC models (not just helicopters) are typically ridiculously overmotorized compared to their real-life counterparts.

And now you know why icefoxes are bigger than desert foxes ;-)

There just aren't many helicopter aerobatic pilots around.
There is one I know of: Chuck Aaron he can do the typical maneuvers in a specially modified helicopter.

i really enjoyed watching that.  thanks.  is that rocket assist at 2:00?

@erich no just standard sky writer smoke

another video i saw said he was the only qualified/faa-authorized pilot to do helicopter aerobatics.  what ridiculous process is there to get qualified?  who qualified chuck aaron?

Also compounded by the fact that most helicopters are not capable of pulling of those manoeuvres.

Most of those maneuvers look like they were done so high that they may not be easily visible from the ground at an airshow, so it might not be the most entertaining to watch.

First off, the reasons you don't see real helicopters performing literally like that model are that it's impossible to get anything like that power-to-weight ratio on a full-size machine and also because the pilot would black out from the ridiculous G's being pulled.
As for why you don't see a whole lot of... less ambitious acrobatic flying in helicopters, it's possible that certification is an issue.  For example, the British Army Lynx display includes rolls, loops and even back-flips. However, I distinctly remember commentary at an airshow I attended saying that, when the Lynx is retired fairly soon, its replacement won't be certified for any kind of acrobatics.

Are you sure about your last sentence?  It seems that the Royal Navy's Black Cats use the Wildcat.

As ratchet freak mentioned, it is simply not common. Why, you ask?
Airplane aerobatics? Calculated, practiced, somewhat risky, entertaining.
Helicopter aerobatics? Closer to a death wish than airplane aerobatics.
The aerodynamics of helicopters are more complex (think fragile). There aren't as many people willing to do it.


text:
What are good strategies for the "impossible turn"?
For reference, I fly a Cessna 172.
I've read about a couple of different strategies for the impossible turn (attempting to turn back to the airport after engine failure at low altitude).
The first strategy is the one I've heard most commonly on aviation websites and videos: that is, essentially, maintain a best glide and try to make the turn as quickly as possible without stalling.  Unfortunately, a slow turn also means you'll have to turn a little more than 180 degrees to get back to the runway.
The second strategy I've heard is not generally mentioned in the typical safety videos (at least that I've seen). A former flight instructor had mentioned that you could complete the impossible turn by pushing the nose down and using the additional airspeed and downward angle to execute an extremely sharp turn that would keep you closer to the runway; when you come out of the turn, you won't have to worry as much about lining back up with the runway.
Strategy 2 is clearly more risky, at least for someone who has not practiced the maneuver, so that could explain why I don't see it mentioned in most safety videos. I was wondering if anyone here had an opinion on either maneuver (or other options) for executing the impossible turn and, if possible, links to literature explaining it.
Thanks in advance.

Such a maneuver is grossly dangerous. See this question for details.

Welcome to Aviation.SE!  As it's currently written I don't think this question is a good fit for our site.  Can you be a little more specific about what sort of opinions you're looking for? Note that this question already has 10 answers, and perhaps might answer your question as well?

There's a reason it's called the impossible turn... the best strategy is to forget there's a runway behind you and find the safest place in front of you to put the plane down without killing yourself.

That sounds insane unless you have Tucker/Hoover genes.

The already linked question has a link to this study. Its authors seem to have done the calculations and concluded that turn with 45° bank flown on the edge of stall lets you do it with smallest loss of altitude. Which is the point here. Don't forget you still need to glide back to the runway.
As rbp mentions below, flying the turn at edge of stall is extremely dangerous, so in practice you should plan turning around only from altitude from which the turn can be made with limit above stall (and don't forget in 45° bank the stall speed is ~20% higher and quickly increases with more bank) you are confident you can manage to maintain. This increases the altitude above which you should only attempt it but the optimal bank angle remain 45°.
If you used higher bank angle, you'd have to fly faster so you'd still have enough vertical lift to balance your weight. That would mean you'd need more lateral lift (even for the same turn rate the lateral acceleration is proportional to speed) to complete the turn and this lift would come with it's additional induced drag. Drag equals energy loss and the only energy you have at that point is potential energy due to altitude. So such turn would spit you out lower, which is the last thing you need. And note, that this is exactly your second option.
Also note, that due to reasons mentioned by rbp below your margin between stall warning and severe stall is even lower in higher bank, so that's another reason against that option.

I don't give that study credence because in a real airplane an accelerated stall caused by increasing angle of bank at the same airspeed doesn't reliably trigger the stall horn; the wing just drops. You can see the difference in this diagram I made. http://imgur.com/SCSg94r it shows two ways to stall at the velocity/load.

@rbp: That diagram does not explain why it should make a difference. Stall warning devices sense whether angle of attack exceeded certain value (either pressure port on the leading edge, vane on the leading edge or AoA vane on side of fuselage), not changes in speed and load. I do understand the warning might be biased to higher or lower AoA if the sensor is on one side only and depending whether it's on inner or outer side of the turn, but that effect is mentioned in that study as far as I remember.

because the load factor can go up in a split second and the stall horn sensor doesn't have time to react before the stall is fully developed. even if you're doing a max performance takeoff stall straight ahead, the amount of time it takes to lose the airspeed is enough to make the horn go off, so students mistakenly think the horn is the first indication. the easiest way to experience this is to do a Vne-5 dive, and pull up quickly into an accelerated stall. go out and try it. stay coordinated :)

Ok, I can understand that. It would mean the pilot has to learn to tighten the curve very gently to avoid changing the load too fast, right? Well, it does say it took some training to be able to make the turn without stalling.

my quibble is that it says to fly with the horn blaring. if you immediately pitch for best glide at a 45-degree bank (since every second counts), i'm concerned that the pilot might mistakenly increase pitch and bank right up the stall curve.

@rbp: I agree, the advice should probably be to know from which altitude you can make the turn with some reasonable speed reserve. It does not invalidate the point about 45° bank being optimal for altitude loss in the turn and it adds another danger to the asker's option b that would be flown at higher load.

my advice is to have a plan for engine failure, and always fly the plan. for example: on the runway, land on the runway. 0-1000 land straight ahead, 1000-2000, make a 180, 2000+ make a pattern. in the aircraft, make the altitude callouts. and fly the plan.

This is the right answer. Also, remember to turn into the crosswind so you don't have to fly so far back to the center line. This means you need to be able make a slow 45 degree turn right as well as left. One other point is that it's dangerous to practice this at low altitude. Go up high or use a simulator. http://www.ntsb.gov/_layouts/ntsb.aviation/brief.aspx?ev_id=20060706X00882&key=1&queryId=9d02f036-4b79-49f7-b95f-2aefa04feae7&pgno=1&pgsize=50

@rbp: I definitely agree. I was saying that the altitude above which you should plan turning should be such that you can make turn with reasonable margin above stall speed and not such that requires flying the turn on edge of stall as that report mentions.


text:
Can Ospreys land without running engines?
I just have seen a video of a demonstration of an Osprey V-22. While this is an impressive aircraft I wonder whether this might land with failing engines (with the large "propellers" probably hinder proper gliding).
What would a pilot do in the following cases:

one engine failed
both engines failed
tilt mechanism stuck

Possible duplicate of Can the V-22 autorotate?

Welcome to AviationStackExchange and to provide a short answer to your question, yes there are autorotation measures that are similar to those featured on helicopters. The basis for most of my answers pertain the unique drive shaft system, which allows for redundancy between proprotors, something other twin-engine fixed wing aircraft usually lack, so here's a diagram for some reference:

As for the detailed answer, I'll split it into three parts: One engine failure, total engine failure, and a stuck rotation mechanism.

The two RR T406 type engines of the V-22 are connected by a driveshaft, which means that if one engine loses power, the other, provided that it is in good working order can deliver power to both proprotors through said shaft. It lacks the power to actually hover on that one engine though, so it has to be glided down, and protocol presumably dictates a powered glide to somewhere safe.
When both engines fail, autorotation just as on a helicopter is instructed, however, due to the fact that the proprotors are about a third of the diameter of conventional rotors means its low rotational inertia results no autorotation will significantly reduce the impact from a total failure. In other words, it can't really glide like a plane and won't autorotate like a helicopter. It's getting the best and worst of both worlds.
As long as the engines are working, the V-22 can land and take off in both configurations. However, there's an approximately 18-20% (Wikipedia quotes 18, Boeing claims 20) increase in flight efficiency in the turboprop aircraft configuration over the VTOL setting. I can only imagine that number goes higher by 10-15% during takeoff and landing. Planes are simply more efficient this way.

So, yeah.

By 'rotation mechanism' I actually meant the ability to tilt the nacelles, for change between the plane/helicopter configurations.

That makes more sense. Anyways, it's my 3 (alternate) one so i'll probably just delete the former. Thanks for clarifying!

I'm wondering how a V-22 would take off in turboprop mode, given that its proprotors extend further down than its wheels do...

I suppose you're right on that count. Though, the actual toggle for the nacelle rotation also can be adjusted manually, so if it comes to it, and there is no available repair then I suppose they could put it somewhere half-way? Talking to some seniors of my afrotc det a lot of emergency procedure they go through includes last-ditch adjustments, especially for landing protocol so hopefully this issue wouldn't completely render a V-22 useless lol.


text:
What is the difference between a 'live' NLG axle and a 'fixed' NLG axle?
What are the advantages / disadvantages of a live and fixed nose landing gear axle and how do they work? Maybe certain aircraft need a particular type fitted?
I can't see the need of a moving axle (assuming that's what 'live' means) unless it is somehow powered by a motor or something similar for push back..
Or..
Maybe on an aircraft a live axle is one where it rotates with the wheel and a fixed one is where the wheel rotates around the axle?
The reason I ask is that I saw in a textbook that nose wheels are either on a fixed or live axle and nose wheel shimmy is prevented by using a twin nose wheel set up attached to a live axle.
I'm now not sure if the terms live and fixed axle are in relation to the vertical component or horizontal component of the nose gear.

A live axle transmits power to the wheels.  I know of no aircraft with such a powered nose landing gear.  Can you give us some context around why you are asking so we might better understand the root of your confusion?

@ Michael Hall: I think - extrapolating from automotive use, since I've never heard the term used in aviation - a live axle would be one with two or more wheels that move independently.  As you say, powered axles don't make sense for aircraft.

A live axle is simply one that moves in some fashion.  On a nose gear, I would assume it to mean the axle is fixed to the wheels and rotates with them, and the bearing interface is between the NG strut and axle.  I've never seen such a thing on an airplane myself.  Maybe the Soviets did it?

Hi John, on most aircraft are nose wheels usually fitted to a fixed axle and the bearing being the interface between wheel and axle?

A fix Nose Landing Gear is a landing gear where the vertical axe between the aircraft and the wheel cannot rotate. The wheel can rotate of course on an horizontal axe (it is how it moves on the ground), but it can not rotate on a vertical axis. This mean that if you try to change your direction on the ground, the wheel will remain in the axis of your current direction, and it will drift when you turn.
A live NLG means that the wheel could rotate on a vertical axis, compared to the direction currently held by the fuselage of the plane. Thus, it is easier to turn when rolling on the ground, but when the airplane takes off or lands, it needs an anti-shimmy device (usually, as you said, a twin nose wheel) to perturbations on the landstrip to make the plane turn hard.
An example of such a thing is when you have a supermarket caddie, and the forward wheels are blocked. It is difficult to turn, but you still can if you put enough torque to the center of gravity of the caddie. On an airplane, having motors on the wings gives easily enought torque on the middle axis of the plane.

Thanks for the input.. Just so I understand you correctly when you talk about fixed and live NLG on the vertical axis this is in relation to the nose landing gear leg /oleo strut and not the horizontal axle? Are you saying a fixed leg (vertical axis) can't turn on the ground? I find that hard to believe, it must have some steering?.. I understand how NLG steering works if this is what you call a 'live' system. My original question was in relation to the horizontal axle being fixed or live.. say for example on a twin wheel NLG.. Some clarification would be much appreciated.

Not only are these definitions incorrect, but I can't think of any airplanes with nosewheels where the gear can't turn left & right. Some are steerable and some are free-castering, but they can all turn. A nosewheel that can't turn would be nearly impossible to taxi with and would probably blow the tire when landing in any kind of crosswind.


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What is the difference between an aircraft main battery and standby battery?
I have just seen a diagram where it shows both a main battery and a standby battery however I am unsure what the differences are?

I thought the main battery was there as an emergency power source / APU starting etc.. so what is the standby battery there for? Or is it a secondary back up?

Depends on the aircraft.
In most piston driven aircraft, the main battery is there for starting. The main battery also sort of runs all electrical equipment, lights and avionics. The only thing it does not run is the ignition system of the engine. I say “sort of” because the alternator supplies the battery and aircraft with more than enough electricity to supply the aircraft’s needs and recharge the battery simultaneously and at a slightly higher voltage. If the alternator goes out, it should not interrupt the total supply of electricity to the aircraft systems.
Think of the battery as a bucket with a hole in it, placed under a faucet (the alternator/generator). As long as the faucet is open enough to supply more than the bucket needs to remain full, you are getting all of the faucets water. If you partially or fully close the faucet, you are only going to get the water that comes from the bucket. But, your water flow is never completely interrupted.
On the other hand, the standby battery is a separate, isolated supply of electricity. As long as the main battery/alternator combination is supplying electricity, the relay is self supporting to keep the circuit closed to supply electricity from the main battery to the aircraft’s Bus Bar (electrical connection to the aircraft’s systems). If the main battery/alternator combo is not supplying electricity because it is dead, the relay will open the circuit from the main battery. It will simultaneously close the circuit supplying electricity from the standby battery to the Bus Bar. This would happen if the alternator/generator was offline or inop long enough to drain the battery.
The relay is probably a spring and electromagnetic set up. The state of being ON makes the main battery active. The state of being OFF makes the standby battery active.
Think of it like the faucet and bucket in the previous example, if the bucket were attached to a spring loaded shut off valve for another faucet. That second faucet was being fed by a container of water. It also had a hose bypassing the bucket. Once the bucket became light enough, the second faucet would turn on to supply you with water. At least, temporarily in a no water emergency.

In the picture above, the piece of electrical equipment in the red dashed box is a relay.  Relays work when electrical power is supplied to the relay and magnetism pulls the relay down.  That is the reason why there are coils (circles) on the bottom of that box.  In this picture, the relay is open connecting the standby battery to the bus bar.
The standby battery has a switch called NORM / EMERG.  That switch is currently in the NORM position preventing emergency battery from connecting to the bus bar.
The main battery also has a switch called ON / OFF and it is in the OFF position preventing the main battery from connecting to the bus bar.  Notice that the main battery powers the relay and when the switch is ON the relay closes and main battery power is routed to the bus bar.
All aircraft that I fly require main batteries to start the airplane before it's APU, GPU or engine driven generators will come online.  The main batteries will also power selected electrical equipment in flight if all the generators fails.  In this situation, the pilot will select EMERG on the NORM / EMERG switch to supply emergency battery power to the bus bar so that certain equipment can still be powered.  In most aircraft, that equipment is the standby attitude indicator, possible tachometer gauges or other engine power gauges, possible landing gear and flaps.  It is all dependent on how that airplane was designed.


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What hardware and software is needed to put together a basic flight simulator?
Due to financial impediments (other priorities in life) I had to postpone my PPL. While browsing the questions on the site here, I was having a look at this question: Can Microsoft Flight Simulator help me learn to fly (or make me a better pilot)?
In that question there is an answer by abelenky where he posts a picture of his old simulator rig. I was wondering about the process to put a similar simulator rig together.
What hardware and software would I need? I've heard about joysticks, but is it necessary to have pedals to move the rudder? To make things as specific as possible, here's how I imagine using it:

The purpose of the simulator
I would like to practice the use of the instruments while enjoying myself. Maybe flying some of my favourite second world war planes for instance, if possible.

What types of planes and flying is it for?
I would see what is available. I have been flying only Cessnas so far. I would love to fly a Focke-Wulf Fw 190 if it's possible.

What types of training or practice?
Navigation, landing, instruments, anything to help with the PPL.

(This is a similar question; it has been closed because it was too broad, since it asked about all possible solutions. I'm only asking which are the essential components to make a basic simulator for PPL-level skills.)

I'm voting to close this question as off-topic because it is essentially asking about sourcing of items, which does not meet our on-topic criteria.

I think this is too opinion-based. Everyone has their favorites.

Have to agree, this is very opinion based on not a great fit for this particular site.  That being said, if you do some google searches you can find some great answers to this question.  Also, check out "The Jay" by Redbird, it has most of the stuff you need in one nice neat little package: http://simulators.redbirdflight.com/products/jay

It has been edited to remove the opinion-based part well before it got closed. I don't see why, in its current state, this would not be a good question.

I still see a lot of opinion going into what exactly "suitable" or "decent" would mean. It would help to be explicit about exactly what the purpose of the simulator is. What types of planes and flying is it for? What types of training or practice?

If you want to see a very basic (i.e. minimal) flight simulator, the source code of one is available here. It won the International Obfuscated C Code Contest (IOCCC) in 1998.

(first off, I need to post a new picture of my sim rig soon.  It's improved since then)
1. Yoke vs. Stick
Try to model it after a plane you're actually going to fly.

If it's a yoke-controlled model (like a Cessna), then you should have a Yoke.
If you're working towards a stick controlled plane (like a Piper Cub), then you should get a simple joystick (not a fighter-pilot-styled super fancy one).

I have this model:
But I want to upgrade to:
2. Throttle
After that, I think a throttle is important.
Making the throttle precisely mirror your chosen airplane is not as important.
(pushing a lever is pretty similar to pushing a control rod).
The three-lever model comes with the Pro Flight Yoke.
Or you can get a a Push-rod version like this:

(more updates when I have time)

I have pedals, but they aren't a key-piece of equipment for most of the basic flying I do.

Can you expand on why you want to upgrade to the Cessna yoke? I too currently have a Saitek so I'm curious of the differences.

I'm trying to model a Cessna 172, so the Cessna Yoke would be more accurate.  Also the green-backlight on the clock in my current yoke doesn't work right, and flickers a lot, which is annoying.  I bought the original yoke before the Cessna model was available, and it is (admittedly) a minor tweak.  I can't really justify the expense, but I still think about it. :)

Well, if you aren't really using the clock anyway, you could just place a layer or two of duct tape over it... That should remove the distraction from the flickering, at a significantly lower cost. :-)

Why do you say the rudder pedals are not essential? If you're modeling an airliner with a yaw damper I could see, but it seems like if you're modeling a 172 you'd want them.


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In the rare event that the U.S. DOD shuts off the GPS, what's the alternative?
In the U.S., the Department of Defense along with other Federal Agencies of the Government operates & maintains the GPS.
In the rarest of the events, if for any reason whatsoever, the Government decides to shut off GPS provisioning (even temporarily), what are the consequences/alternatives for the aviation industry?

What percentage of the industry solely relies on The GPS to operate?
What percentage of the industry has enough alternative navigation means to continue operation regardless The GPS?

On a broader spectrum, I'm also curious about the aviation industry's reliance on GNSS and what could happen in cases of global disaster, wars, cyber-wars etc. (all of which are not impossible).

GPS is unavailable regularly because of military testing or other reasons. There are GPS NOTAMs to inform pilots.

@Pondlife throughout the country at the same time?

No, but it's still enough to affect navigation in those specific areas.

The American GPS is not the only GNSS system in the world. See https://en.wikipedia.org/wiki/GLONASS or https://en.wikipedia.org/wiki/Galileo_(satellite_navigation) or https://en.wikipedia.org/wiki/BeiDou_Navigation_Satellite_System

@J.Hougaard yes, and that’s why my question was specific to The U.S.

@RaajTram Those systems also cover the U.S.  The problem is that in a crisis where turning USGPS off is reasonable, it's likely those other civilized nations will also cooperate by shutting theirs off.

@Pondlife: GPS is not "unavailable regularly". That is complete nonsense.  Individual satellites periodically undergo maintenance, and some ground-based auxiliary services might periodically undergo maintenance. But the GPS constellation as a whole has been operating uninterrupted for decades. When half the constellation temporarily reported the wrong UTC correction in January 2016 it was deemed a hugely disruptive and unusual event. Besides, the military signal is different from the civilian signal and you are never using it (for one, it's encrypted so that you can't).

It's actually not particularly implausible that the civilian GPS signal could be disabled, when you consider particular war cases. As described by others here, though, there are plenty of alternatives. The real problem is that most infrastructure is yet to make any use of those alternatives.

@LightnessRacesinOrbit GPS is occasionally unavailable in certain areas. Example You're right, of course, that entire system unavailability is not by any means common.

@LightnessRacesinOrbit You're correct that GPS is not regularly turned off. On the other hand, it is regularly unavailable for air navigation use in specific areas in the US, usually because of military interference testing. As a pilot, hearing that "GPS is always available, you just can't receive it sometimes" doesn't help me much; as a practical matter, the DOD regularly decides to make GPS unavailable in defined areas. Hence the need for GPS NOTAMs. I don't know if that fits your definition of "unavailable" or not.

Trains will work.  Railways have never used it, and even though they are migrating to a new train control system, they are going through backflips to avoid relying on GPS etc.  They already have automatic signaling (akin to the ground lighting now going in on airfields), the new system will override the engineer and do a lot of other cool stuff.

If the GPS is unavailable, it will be quite an impact to the aviation industry.
All airliners in-flight will experience degraded RNAV performance, but they would make it to the destination using VORs, DMEs and ILSs. For general aviation, things are not so lucky. The GPS display provides an excellent situation awareness in small aircrafts; without it, navigation reverts to old school VFR and reading charts. Most pilots will probably make it, but a few may get lost and end up in fuel exhaustion.
For airliners preparing for departure, initiating the INS is time consuming as the GPS coordinate is used as one of the startup parameters. The coordinates of the gates can be looked up in airport charts, some airports also have signs which specify the GPS location; both of these take longer time. Suddenly, pilots flying oceanic routes find out they need more planning for contingencies. Some routes may be changed to fly VOR-to-VOR. Departures around the world are going to be delayed, that is for sure.
Ground Proximity Warning Systems, which use GPS to determine the aircraft's location, are unable to use their terrain database to provide warnings to pilots, although warnings based on radar altimeters should still be available.
Basically, we are thrown back to the 80s or 90s, when GPS was not yet widely adapted. Many of those navigation aids are still in use today, so the industry can still fly, but traffic capacity will be reduced.

”Basically, we are thrown back to the 80s or 90s...”. So, new Embry Riddle Graduate Pilots won’t get jobs? What else is new?

Well, there's always ATC. "Help, I'm lost, what's my position?"

With the current VOR deprecation schedule, falling back won't be viable for much longer.  It's marginal already...

What is a GPS coordinate and how does it differ from geographic coordinate, INS coordinate or GLONASS or Galileo coordinate?

Dude, no. Pilots are trained for this. It's a slight inconvenience because direct-to routings and GPS-only approaches are unavailable, but if you get lost without GPS you have no business operating a plane.

Are you sure GPWS terrain database would actually stop working rather than using VOR/DME + INS + any other remaining positional guidance systems in the FMS to figure out where the aircraft is relative to the terrain database? That seems like it would be an odd design.

Also, why would GA not use VORs, DMEs, and ILSs like airliners? Are there really that many GA aircraft that aren't equipped for such? And, if so, it seems like that they shouldn't have any business flying outside of VMC, anyway. And, of course, in VMC, eyes and a chart are generally quite sufficient for determining one's position.

@reirab I am fairly certain that GPWS uses GPS, such that it is immune to INS errors, altimeter errors (e.g. static port blockages) and NAV outages. Regarding GA, it is not the aircraft, but the pilots. A small minority of GA pilots operate with "not so good" airmanship, and this is reflected by the number of GA accidents that could have been prevented.

@kevin Yes, I'm sure it uses GPS, I just meant that I'd assume it would fall back to whatever positional data sources the FMS had available if GPS became INOP rather than degrading to radio alt only.

Is GLONASS or Galileo not (yet) used in aircraft?

@kevin Airbus's EGPWS uses IR (1) position, so your milage might vary ..

@VladimirF I think "GPS coordinates" just means "latitude and longitude" and the use of phrases like this is just an example of how dependent on GPS we've become. In common parlance, we refer to the thing that GPS "measures" as "GPS". It's almost as if we asked people "What's your wrist-watch?" instead of "What time is it?"

@VladimirF a "GPS coordinate" would be a latitude and longitude taken from the WGS84 reference system

@dkwarr87 Yes, but even there, GPS (as an engineering system) has nothing to do with WGS84 (which defines coordinates). The "positioning" in GPS means "to determine position", not "to define position".

@SimonRichter : trained, maybe. but they don't have the same experience with it as pilots in the 70's who used it in every flight.

There are going to be bigger issues. The shipping industry is GPS reliant, the communications industry heavily relies on GPS time signals, Routing for trains, and so on.  The fact is that the chaos without GPS will extend far beyond aviation. It will impact the network, the power grid, agriculture, mining, and so on.
In 1992, I had a team which proposed a smaller aircraft launched GPS system in LEO to handle such an emergency. For the most part, receivers could use the LEO transmitters.
But if I were just worried about aircraft, then I would also have GLONASS, Galieo, and others integrated into the receivers. I believe many receivers are currently configured to handle GPS and GLONASS concurrently, and employ WAAS, and the European, Japanese and Indian augmentation systems as well.
Things are changing, because I recently found a GPS/GLONASS receiver with WAAS and the other equivalents, with a USB interface, for $18. Pretty amazing.

This raises a question - would it be wise should ALL the GPS receivers be manufactured such that they accept GPS signals from all the global GNSSs?

Depends upon your market.  Or are you thinking legislatively?

@RaajTram: Russia already  demands GLONASS (can't sell pure GPS there) and Galileo is designed to be highly GPS(NavStar) compatible. The Chinese, Japanese and Indian systems are not global.

Currently IFR capable aircraft are required to retain VHF navigation radios for use with terrestrial navigation beacons.  These can work with existing RNAV systems as well.
Other options for navigation from a failure of the GNSS service can include the following.
LORAN - still popular with maritime operations, LORAN is still a very accurate means of navigation.  Unfortunately aviation LORAN receivers fell out of favor with the advent of GPS.
Inertial Navigation Systems - current generations of solid state INS gear are highly accurate but accuracy begins to degrade with prolonged use, requiring accurate position updates from time to time.
Celestial Navigation systems - electronic sextants which can track known stars in the sky.  The systems can be combined with other INS and RNAV systems for improved performance and redundancy.

+1, but can you cite a FAR or other reference for the claim in your first sentence? IIRC the only requirement is equipment suitable for the route to be navigated.

There are a few working LORAN transmitters worldwide, but just a few... Probably too few to give a planetary coverage...

LORAN was officially shut down back in 2010...

According to the AIM (US) your first statement is incorrect. "Aircraft using [un-augmented GPS] for navigation under IFR must be equipped with an alternate approved and operational means of navigation suitable for navigating the proposed route of flight."  "WAAS avionics are evaluated without reliance on other navigation systems. As such, installation of WAAS avionics does not require the aircraft to have other equipment appropriate to the route to be flown." As a practical matter, I doubt that there are any aircraft that are GPS/WAAS equipped that do not also have the ability to use VORs.

No VHF Navigation Radios are required for an IFR capable airplane with or without GNSS equipment aboard.

In the past, the US GPS system incorporated Selective Availability (SA), this ensured civilian users has less accurate positioning than the US military. SA was switched off in 2000, and since 2007, new GPS satellites do not have the option to switch on SA.
If the US military needs to degrade GPS service, it'll be done in a limited region. Presumably that region will be a warzone, and will be closed to commercial and general aviation anyway.
If the US ever has to switch off the entire GPS system, WW3 has probably broken out. Again, navigation won't be the biggest problem for commercial and general aviation in this scenario.

I am not sure about the current distribution in the market, but the value of other constellation owners like GLONASS or Galileo would suddenly increase. Currently, due to wide usage of GNSS they aren't getting much traction. In fact, the majority of cellphones currently have both GNSS/GLONASS available. In fact, Russia poses additional tax if a device doesn't have GLONASS support to increase the usage of GLONASS. So the only thing I see is a sudden bump in usage of other constellations. My iPhone/Samsung phone would be just fine!

Aviotion does not really use phone satnavs.

@VladimirF: GPS receivers these days use specialized chips to convert the radio signals into longitude/latitude/height coordinates. Further processing will differ between applications (such as navigation), but it's this first chip which will handle GLONASS and Galileo.

authors: Vladimir F Героям слава


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What is this class of bolts called?
They can be seen in many —mainly military— jet engines. Do these bolts recibe an special name?

J79: the fasteners are everywhere

EJ200: they can be discerned in the rear flange

12 point flange cap bolt/screw. For high torque applications, less likely to be accidentally rounded-off.
http://www.fbabolt.com.au/12-point-advantages.html

They are also make from an extremely heat-resistant alloy, so their strength is retained at high temperatures.

I agree that the bolts in question are some variant of the 12 point flange cap bolt, but these bolts appear to be more specialised. I think there may be additional specifications, such as material, grade, and stretch rating. If you look at the shaft these appear to be a form of stretch bolt such as those used in connecting rod applications in piston engines. We may need to consult an engine IPC.